Main uses and habits of Magnolia

Most Magnoliaspecies have economic importance (). They have been extensively introduced and cultivated as ornamental plants (e.g., Magnolia grandiflora and M. coco), timbers with relatively high quality (e.g., Magnolia officinalis var. biloba), medicinal plants (e.g., a famous Chinese traditional medicinal material Flos Magnoliae is from Magnolia liliflora), and natural resources of stacte, a sweet spice used in making incense, and flavour (e.g., M. cylindrica), for a long time (Table Uses and main habits of Magnolia ).

Almost all Magnolia species are valuable planted ornamentals. For example, Magnolia grandiflora is native of the middle and southern sections of Georgia, South Carolina, Alabama, Louisiana, and the upper districts of Florida, USA, and has been extensively planted worldwide. It is a noble urban landscape tree because it is resistant to acid deposition ().

In China, some Magnolia species, e.g., M. elliptigemmata and M. pilocarpa, have become new resources of medicinal Biond Magnolia Flower (Flos Magnoliae), besides Magnolia liliflora (). Phytochemical studies of Magnolia have reported that the leaves, fruits, barks, and woods of many species, e.g., Magnolia grandiflora, yield a variety of extracts with potential applications as pharmaceuticals ().

Nearly two-thirds of commercial Magnolia species can be used in making furniture products (). The sapwood of Magnolia is creamy white, while the heart-wood is light to dark brown, often with greenish to purple-black streaks or patches. The high-quality wood of Magnolia is even-textured and moderately heavy, fairly hard and straight grained. It is resistant to heavy shrinkage, is highly shock absorbant, and has a relatively low bending and compression strength. It takes glue well, has a good nailing quality, and stains and varnishes easily (). Magnolia wood is also used by the food industry for making cherry boxes, flats, and baskets, and is used for popsicle sticks, tongue depressers, broomhandles, veneers, and venetian blinds, in North America ().

Magnolia may be important to livestock and wildlife. For example, Magnolia virginiana is one of the major forages for deer and cattle (). Winter use by cattle can account for as much as 25% of their diet (). White-tailed deer browse the leaves and twigs year-round (). The seeds of Magnolia virginiana are eaten by gray squirrels and to a lesser extent by white-footed mice, wild turkey, quail, and song birds (, and Magnolia grandiflora by squirrels, opossum, quail, and the wild turkey ().

In addition to economic significance mentioned above, some rare and endangered Magnolia species are important in conservation biology. For example, M. sharpii is endemic to the state of Chiapas, Mexico, and is known to occur in only five small fragmented populations. M. schiedeana is also endemic to Mexico and is listed as threatened with extinction.

The main habits or botanical characteristics of 53 taxa of Magnolia are also given in Table Uses and main habits of Magnolia. In general, they are evergreen or deciduous arbors or shrubs. Most arbor species (e.g., M. denudata and M. cylindrica) are middle-tall trees in evergreen hardwood forests or evergreen deciduous-hardwood mingled forests. Some trees can reach heights of 24 m, with a diameter of 1m. A wild tree of M. denudata found in Mt. Yuntai, Jiangsu, China, is approximately 1000 years old ().

Most Magnolia species are temperature-sensitive trees. The North American natives bloom in early summer after the leaves expand, and many of the Asian species bloom in early spring on naked twigs (). The blooming date of a Magnolia species varies according to the climate in which it is located. For the same species in different locations or different species in the same location, the dates may differ by up to 4 or 5 months. The annual breeding periods of Magnolia species may also be different. Some species, such as M. denudata cultivated in Shanghai and southeastern China, have a relatively long breeding period within a year: flowering in early March, leafing after flower dropping, floral initiation in the end of June, fruit setting in September, leaf color changing in October, and leaf dropping in November. Seed-picking time of Magnolia ranges from September to November.

The flowers of Magnolia species are terminal, solitary, large, and often showy (). Pollination is facilitated by beetles feeding in the flowers ().

Management of cultivated Magnolia

Fertilizer

Most woody plants have comparatively low nutrient requirements, and cultivated plants of Magnolia have only normal requirements for fertilizers. However, phosphate fertilizers should be used in sour soils, especially before the periods of blooming and fruit setting of Magnolia (Law, 1990).

When applying fertilizer, it should be borne in mind that the majority of the feeding roots of Magnolia are beneath the drip-line of the outer branches and not close to the base of the bush or tree. Magnolia species are surface rooting, so on no account should the ground be dug over beneath their branches ().

Irrigation

Winter drought can cause extensive die-back and mortality of most Magnolia species, such as Magnolia grandiflora. Irrigation and water retention are also necessary for Magnolia during summer. Where watering becomes advisable, the irrigation should be prolonged and copious.

Pruning

Magnolia requires no routine pruning but it may be pruned; in particular, newly planted trees may be cut down to ground level. Removal of at least half of the mature leaves is a pretransplanting recommendation for rooted layers of the trees, such as Magnolia grandiflora and M. delavayi.

Where a strong basal shoot arises on young trees of Magnolia growing on their own roots, the original growth may be cut away in March or April. Some species tend to produce long, gangling branches and it may become desirable to shorten these, especially where space is limited. The best times to do this are immediately after flowering, or in July or August ().

A simple method of root pruning for some Magnolia was suggested by Cyril H. Isaac (). In this it is recommended that a circle be marked out about 1m from the trunk. A sharp spade is then forced down to its full depth to sever all roots in its path. Then, missing a spade’s width, the operation is repeated right around the tree. This method may be useful for restricting the growth of the larger types and reducing the time taken for a tree to begin flowering.

Diseases and pest control

Most Magnolia species have relatively strong disease resistance. Die-back of shoots and branches can be attributed generally to a natural overcrowding of the branches, combined with the relatively large size of most Magnolia leaves. Secondary fungal organisms soon prey upon these overcrowded growths and may resemble a primary infection. For example, a number of fomes and polyporus fungi cause heartrot in Magnolia grandiflora. Leaf spot, sometimes spreading over the surface of isolated leaves in more-or-less concentric rings, has been attributed to the common gray mold fungus (Botrytis cinerea), and also to Phyllosticta species. Cankers, occasionally met with in Magnolia, are generally attributed to infection by Phomopsis species. They rarely occur where the basic requirements of fertile, not-too-alkaline soil, adequate drainage, and suitable climatic conditions prevail ().

Moles and occasionally field-mice and ants have been known to jeopardize recently planted Magnolia by burrowing beneath their roots and thereby loosening and drying out the soil. It is a good idea to water the compacted area copiously prior to raking over with loose soil or mulching materials.

Mealy-bug will attack Magnolia grown perpetually under glass. A simple method of control is to dab paraffin or methylated spirits on to the mealy covering beneath which the insects are hidden. Greenhouse red spider (Tetranychus telarius) can cause considerable damage to the leaves, especially of deciduous Magnolia species, when grown under glass. Heavy infestations of magnolia scale (Neolecanium cornuparyum) have been reported (). The extent of their natural protection makes it very difficult to eradicate them by spraying. Scales of various types will infest twigs. Overwintering scales can be controlled with dormant oil applied in the spring. Where fumigation is feasible a smoke generator or electrically heated fumigator is recommended ().

Microculture of Magnolia

The potential utilization of microculture in the propagation and study of Magnolia is considerable, but this potential has yet to be realized. The in vitro performance of Magnolia is often poor relative to that of most other microcultured plants, largely owing to the absence of optimized in vitro methodologies.

Protocols for the rapid and reliable cloning of Magnolia genotypes are critical to efficient micropropagation and would be an important component of a biological or phytochemical study. These techniques would provide the consistent tissues necessary for replicated analyses. Genotypes of interest can be established in microculture as sterile shoot cultures from which uniform organs or shoots can be obtained independently of seasonal variation.

In general, Magnolia species possess high levels of phenolic compounds (). Phenolic exudates are a recurring problem in the microculture of many woody plants (), and likely reduce the efficiency of Magnolia microculture. Magnolia species also possess relatively large in vitro shoots and leaves, a fact that greatly limits the number of shoots that can be grown per microculture vessel.

The rooting and establishment of Magnolia microcuttings appears to be a greater problem than is in vitro multiplication. The rooting of Magnolia cuttings is often difficult (), but some studies have shown encouraging results. Maene and Debergh () found that supplementing established cultures with liquid media prior to ex vitro rooting significantly increased rooting percentage. Kamenicka () analyzed the effects of different carbohydrate sources in the media and found that fructose, mannose, and xylose promoted the best in vitro rooting over a period of 13 weeks.

Most reports of Magnolia microculture involve the establishment and multiplication of genotypes on conventional media/hormone formulations utilized for a range of plant genera. The definition of media/hormone responses, particularly cytokinin response curves, is critical to the development of an effective microculture protocol. Biedermann () cultured several genotypes of Magnolia on a number of media formulations and found that Magnolia performed better on media containing relatively low salt concentrations. This suggests that the widely used Murashige and Skoog (MS) () medium may be inferior to lower-salt media such as Anderson’s Medium (AR) () and Woody Plant Medium (WPM) () for Magnolia microculture. In China, the first reported medicinal Magnolia in vitro was Magnolia officinalis, for which a modified MS medium was used ().

The aforementioned concerns indicate that shoot culture may not be an effective means of generating large numbers of a Magnolia genotype. However, substantial numbers may be possible through somatic embryogenesis. Work with Magnolia virginiana, M. fraseri, and Magnolia acuminata () indicate that somatic embryogenesis could potentially perform this role if Magnolia genotypes are needed in great quantity.

Another potential application of microculture in Magnolia propagation may be to maintain stock plant juvenility. As an alternative or supplement to micropropagation, microculture may serve as a means of supplying highly juvenile stock plants from which quality softwood cuttings can be obtained. The juvenility obtained from in vitro culture can improve stock plant vigor and the rooting percentage of cuttings ().

The problems related to Magnolia microculture should not be a deterrent to developing efficient in vitro systems for this genus. Economically feasible systems have been developed for other woody plants that were initially recalcitrant in vitro, including Kalmia () Rhododendron () and Syringa (). These three genera are now in widespread commercial microculture. Other woody genera for which optimized in vitro methodology has been developed and which are now commonly produced through micropropagation include Amelanchier, Betula, Populus, and Ulmus ().

The relatively modest success achieved to date in the microculture of Magnolia holds significant promise for the future application of this technology to the propagation and study of this genus. Thorough investigation to determine the optimum protocol for in vitro establishment, multiplication, and rooting will be required before the widespread microculture of Magnolia becomes a reality.